Parton energy loss in QCD matter

QCD jets, produced copiously in heavy-ion collisions at LHC and also at RHIC, serve as probes of the dynamics of the quark-gluon plasma (QGP). Jet fragmentation in the medium is interesting in its own right and, in order to extract pertinent information about the QGP, it has to be well understood. We present a brief overview of the physics involved and argue that jet substructure observables provide new opportunities for understanding the nature of the modifications.Comment: 6 pages, 1 figure; plenary talk at the 8th International Conference on Hard and Electromagnetic Probes of High-energy Nuclear Collisions (Hard Probes 2016), Wuhan, China, September 23-27, 201

Dissipative Axial Inflation

We analyze in detail the background cosmological evolution of a scalar field coupled to a massless abelian gauge field through an axial term $\frac{\phi}{f_\gamma} F \tilde{F}$, such as in the case of an axion. Gauge fields in this case are known to experience tachyonic growth and therefore can backreact on the background as an effective dissipation into radiation energy density $\rho_R$, which which can lead to inflation without the need of a flat potential. We analyze the system, for momenta $k$ smaller than the cutoff $f_\gamma$, including numerically the backreaction. We consider the evolution from a given static initial condition and explicitly show that, if $f_\gamma$ is smaller than the field excursion $\phi_0$ by about a factor of at least ${\cal O} (20)$, there is a friction effect which turns on before that the field can fall down and which can then lead to a very long stage of inflation with a generic potential. In addition we find superimposed oscillations, which would get imprinted on any kind of perturbations, scalars and tensors. Such oscillations have a period of 4-5 efolds and an amplitude which is typically less than a few percent and decreases linearly with $f_\gamma$. We also stress that the comoving curvature perturbation on uniform density should be sensitive to slow-roll parameters related to $\rho_R$ rather than $\dot{\phi}^2/2$, although we postpone a calculation of the power spectrum and of non-gaussianity to future work and we simply define and compute suitable slow roll parameters. Finally we stress that this scenario may be realized in the axion case, if the coupling $1/f_\gamma$ to U(1) (photons) is much larger than the coupling $1/f_G$ to non-abelian gauge fields (gluons), since the latter sets the range of the potential and therefore the maximal allowed $\phi_0\sim f_G$.Comment: 22 pages, 27 figure

Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung

We argue that contemporary jet substructure techniques might facilitate a more direct measurement of hard medium-induced gluon bremsstrahlung in heavy-ion collisions, and focus specifically on the "soft drop declustering" procedure that singles out the two leading jet substructures. Assuming coherent jet energy loss, we find an enhancement of the distribution of the energy fractions shared by the two substructures at small subjet energy caused by hard medium-induced gluon radiation. Departures from this approximation are discussed, in particular, the effects of colour decoherence and the contamination of the grooming procedure by soft background. Finally, we propose a complementary observable, that is the ratio of the two-pronged probability in Pb-Pb to proton-proton collisions and discuss its sensitivity to various energy loss mechanisms.Comment: 20 pages, 5 figure

Parton energy loss: new theoretical progress

The physics of jet quenching combines the dynamics of the QCD parton shower with bremsstrahlung radiation and decoherence processes induced by interactions with an underlying medium. Here we present a brief overview of the established features of medium-induced bremsstrahlung spectrum in a deconfined QCD plasma, highlight the aspect of rapid jet showering inside the medium and compute the resulting energy lost out of the jet cone in heavy-ion collisions.publishedVersio

Jet (de)coherence in Pb-Pb collisions at the LHC

We study the modifications of jets created in heavy-ion collisions at LHC energies. The inherent hierarchy of scales governing the jet evolution allows to distinguish a leading jet structure, which interacts coherently with the medium as a single color charge, from softer sub-structures that will be sensitive to effects of color decoherence. We argue how this separation comes about and show that this picture is consistent with experimental data on reconstructed jets at the LHC, providing a quantitative description simultaneously of the jet nuclear modification factor, the missing energy in di-jet events and the modification of the fragmentation functions. In particular, we demonstrate that effects due to color decoherence are manifest in the excess of soft particles measured in fragmentation functions in Pb-Pb compared to proton-proton collisions.Comment: 5 pages, 2 figure

Towards tomography of quark-gluon plasma using double inclusive forward-central jets in Pb-Pb collision

We propose a new framework, merging High Energy Factorization with final-state jet quenching effects due to interactions in a quark-gluon plasma, to compute di-jet rates at mid- and forward rapidity. It allows to consistently study the interplay of initial-state effects with medium interactions, opening the possibility for understanding the dynamics of hard probes in heavy-ion collisions and the QGP evolution in rapidity.Comment: 10 pages, 5 figure

Universal scaling dependence of QCD energy loss from data driven studies

In this paper we study the energy loss of jets in the QGP via the nuclear modification factor $R_{\textrm{AA}}$ for unidentified particles at high $p_{\textrm{T}}$ ($\gtrsim 10 \textrm{GeV}/c$) in and out of the reaction plane of the collision. We argue that at such a high $p_{\textrm{T}}$ there are no genuine flow effects and, assuming that the energy loss is only sensitive to initial characteristics such as the density and geometry, find that $R_{\textrm{AA}}$ depends linearly on the (RMS) length extracted from Glauber simulations. Furthermore we observe that for different centrality classes the density dependence of the energy loss enters as the square root of the charged particle multiplicity normalized to the initial overlap area. The energy loss extracted for RHIC and LHC data from the $R_{\textrm{AA}}$ is found to exhibit a universal behavior.Comment: 15 pages, 5 figures, version to be published in Phys. Rev.

Jet formation and interference in a thin QCD medium

In heavy-ion collisions, an abundant production of high-energy QCD jets allows to study how these multiparticle sprays are modified as they pass through the quark-gluon plasma. In order to shed new light on this process, we compute the inclusive two-gluon rate off a hard quark propagating through a color deconfined medium at first order in medium opacity. We explicitly impose an energy ordering of the two emitted gluons, such that the "hard" gluon can be thought of as belonging to the jet substructure while the other is a "soft" emission (which can be collinear or medium-induced). Our analysis focusses on two specific limits that clarify the modification of the additional angle- and formation time-ordering of splittings. In one limit, the formation time of the "hard" gluon is short compared to the "soft" gluon formation time, leading to a probabilistic formula for production of and subsequent radiation off a quark-gluon antenna. In the other limit, the ordering of formation is reverted, which automatically leads to the fact that the jet substructure is resolved by the medium. We observe in this case a characteristic delay: the jet radiates as one color current (quark) up to the formation of the "hard" gluon, at which point we observe the onset of radiation of the new color current (gluon). Our computation supports a picture in which the in-medium jet dynamics are described as a collection of subsequent antennas which are resolved by the medium according to their transverse extent.Comment: 33 page